Silicon (Si) is a widely used material in biomedical research1-11 because it is biocompatible and biodegradable, and it exhibits a spectrum of important electrical, optical, thermal and mechanical properties. For example, Si-based systems can sense electrical activities of the brain in flexible and adhesive configurations12-14, deliver nucleic acids in vivo to induce angiogenesis7, and perform intra- and intercellular force dynamics measurements15. So far, most of the applications of Si as biomaterials have been focused on or originated from single crystalline structures or substrates, which is primarily due to the need for high quality and controllable electrical or other properties. Natural biomaterials have remarkable diversity in structure and function and have informed the design16 of new Si forms17-22 for subcellular interfaces and biophysical modulation (FIG. 1A, top). While nano-casting synthesis of mesoporous solids is highly versatile and scalable, ordered and freestanding Si-based mesostructures with molecular-level principal feature sizes (i.e., <10 nm) are still challenging to achieve17,18,27,30.